Background and objectives

Integrated Ocean Drilling Program (IODP) Site U1403 (proposed Site JA-01A; 39°56.60′N, 51°48.20′W, 4949 m water depth) (Fig. F1) is the deepest water site to be drilled on J-Anomaly Ridge and pins the deep end of the Paleogene Newfoundland sediment drifts depth transect. The site was positioned

To capture deep excursions of the carbonate compensation depth (CCD) during the Paleogene and

To help improve stratigraphic control on the sediment that was drilled at J-Anomaly Ridge (Figs. F2,F3).

Because Site U1403 is located at the toe of J-Anomaly Ridge, coring at this site will also capture

A condensed sequence (~150 m thick) through the prominent acoustically transparent package seen in our seismic data that is much more expanded upslope (>700 m thick) and

Prior to drilling, we hypothesized these two packages were middle and upper Eocene drift deposits overlying a sequence of lower Eocene through Upper Cretaceous pelagic sediments. Given its current water depth of nearly 5 km and assuming a subsidence history for J-Anomaly Ridge akin to that reconstructed from Deep Sea Drilling Project (DSDP) Leg 43 results (Tucholke and Vogt, 1979) (see Fig. F2 in the “Expedition 342 summary” chapter [Norris et al., 2014a]), Site U1403 should be a sensitive recorder of CCD fluctuations, particularly during intervals when the CCD was depressed to abyssal depths. Of particular interest are CCD-deepening events associated with “overshoots” in carbonate ocean chemistry in the early recovery phases of the most extreme Cenozoic carbon cycle perturbations, such as the Eocene–Oligocene transition, the Paleocene/Eocene Thermal Maximum (PETM), and the Cretaceous/Paleogene (K/Pg) boundary (Figs. F2,F3). All three of these extreme events are thought to involve increased deep-sea carbonate burial flux involving a rebalancing of oceanic carbonate chemistry following major shocks to the Earth system (Dickens et al., 1997; Zachos et al., 2005; Merico et al., 2008). Site U1403 is positioned to capture a sedimentary expression of these carbonate overshoot events, which should be evident both through increased abundances of carbonate and improved carbonate microfossil preservation. Of particular interest at Site U1403 are early and middle Eocene “hyperthermals,” the PETM, and the K/Pg boundary event.

Seismic stratigraphy (Figs. F2,F3) shows that the acoustically transparent sediment package thins considerably between modern water depths of 4800 and 4900 meters below sea level (mbsl), corresponding to paleodepths at 50 Ma of ~4600 mbsl. At the outset of Expedition 342, we hypothesized this thinning indicated the average position of the CCD during the middle and late Eocene.

We anticipated that the underlying ~350 m thick reflector-rich package of lower Eocene and older sediment accumulated within or below the CCD. The nature of this sediment package is also of great interest for CCD reconstructions because the acoustic expression is of a sediment pile that expands in thickness with increasing water depth, exactly opposite from the overlying hypothesized middle and upper Eocene drift sequence.

Tucholke and Vogt (1979) reconstructed the North Atlantic CCD to be positioned at ~ 3 km paleodepth during the Late Cretaceous, to undergo a temporary depression to below 5500 mbsl in the latest Cretaceous and early Paleocene, and to be located around ~4400–4300 mbsl in the Eocene (see Fig. F2 in the “Expedition 342 summary” chapter [Norris et al., 2014a]). Compilation of carbonate records from North Atlantic DSDP sites, however, reveals that there is little constraint on the depth of the CCD in the Paleocene and Late Cretaceous North Atlantic Ocean. Sequences of Maastrichtian and Paleocene multicolored clay, presumably representing sub-CCD deposits, are present at ~5000 mbsl paleodepth at DSDP Site 386 near Bermuda and at DSDP Site 105 on the Hatteras Abyssal Plain. The upper possible depth range of the CCD in the Maastrichtian and early Paleocene is constrained by pelagic carbonates encountered at 3300 mbsl paleodepth at DSDP Site 384 (Tucholke and Vogt, 1979). Hence, prior to our expedition, the composition of the lower package at the toe of J-Anomaly Ridge at a paleodepth of ~4000–4500 mbsl was extremely uncertain. Based on Paleogene CCD reconstructions for the equatorial Pacific (Lyle, Wilson, Janecek, et al., 2002; Pälike et al., 2012), it is quite possible that the sediment comprising the reflector-rich package at Site U1403 could have accumulated almost entirely below the CCD. Another hypothesis for Site U1403 is that the strong impedance contrasts seen in the upper 120 m of the lower sediment package corresponds to a sequence of alternating clays and cherts. Based on stratigraphic information from Site 384 and drilling results from Blake Nose (Norris, 2001), the particularly bright reflector at ~150 meters below seafloor (mbsf) was interpreted to be a prominent condensed and chert-rich interval of early/middle Eocene boundary age. In this interpretation, the PETM was anticipated at ~230–240 mbsf, associated with a change in reflector character, and the K/Pg boundary at ~300 mbsf.

In addition to playing a key role in CCD reconstructions, Site U1403 was also needed to help understand the history of chemical stratification and ocean currents in the Paleogene North Atlantic Ocean. Most deep ocean drill sites are located at mid-ocean depths (~2000–2500 mbsl) when we account for thermal subsidence. Hence, Site U1403 is rare in that it was selected to recover a representative sequence deposited in true deep water (~4500 mbsl in the Eocene). Furthermore, the site is well placed to record the history of deep waters formed in the far North Atlantic, or even the Arctic, because the western boundary current is constrained to flow directly over or around the Newfoundland ridges by geostrophic flow and the shape of the ocean basin. At the deep end of the J-Anomaly depth transect, the site should record the chemistry and flow history of abyssal waters in the Paleogene.